This proposal aims to address a fundamental issue of biology-how the processes of cell proliferation, pattern formation, and differentiation is regulated in order to generate the complex tissues of an adult organism. This issue is relevant not only to basic research in development biology but also to efforts to understand defects of the mature state such as the deregulated proliferation characteristic of cancer. This proposal specifically investigates the role of a potential new signaling center in specifying the division and differentiation of adjacent cells in the developing Arabidopsis root. This center expressed SHORTROOT (SHR), which, although it encodes a putative transcription factor, has been found to exhibit non-cell-autonomous activity. First, the nature of the signaling molecule will be examined by exploring two alternative hypotheses: (1) that the SHR protein itself is transported into the responding cells, and (2) that shr-dependent signaling is ligand-receptor mediated. Second, the influence of the SHR-expressing signaling on a important stem cells will be assessed relative to the known influence on that cell type by another signaling center, the quiescent center. These goals will be accomplished using techniques such as immunohistochemistry, plant transformation, suppressor screening, and genetic crossing. These experiments aim to provide insights into mechanisms involved in patterning and cell-type specification during organogenesis.